DEFINITION OF QUATERNARY

The last 1.8 million years of Earth history are

known as the Quaternary period, divided into the older Pleistocene

and the younger Holocene. The rocks and unconsolidated

deposits formed during this period were first

recognized by Jules Desnoyers to be different from older

deposits by their characteristic boulder clays and other units

deposited by glaciers in Europe. It was soon recognized that

these deposits reflected globally cool climates for the first

part of the Quaternary, since glacier deposits were found in

many parts of both the Northern and Southern Hemispheres.

Global climate zones were condensed toward the equator,

with ice sheets covering about one-third of the continental

surfaces and desert regions converted to moist grasslands.

Grasses, plants, and mammals experienced a rapid expansion.

Another major important discovery from this period was

the recognition of the first human fossils, which became the

basis for dividing the Quaternary into the older Pleistocene,

and the younger Holocene, in which the human fossils

appear abundantly about 10,000 years ago. Older primate

fossils including early hominids are found in the older record

going back several million years, and the record of human

habitation in the Western Hemisphere may extend back to

13,000 or 14,000 years ago. Genetic evidence suggests that

humans are all descendants of a single female ancestor that

lived somewhere in East Africa about 100,000 to 300,000

years ago, with the first hominids appearing about 4 million

years ago.

See also HOLOCENE; NEOGENE; PLEISTOCENE; TERTIARY.

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